Microelectronic devices are used in cell phones, pagers, personal digital assistants, computers and many other products. A packaged microelectronic device can include a microelectronic die having integrated circuitry, an interposer substrate or lead frame attached to the die, and a molded casing around the die. The molded casing can also encase a portion of the interposer substrate or lead frame. The microelectronic die generally has a plurality of bond-pads coupled to the integrated circuitry. The bond-pads are coupled to terminals on the interposer substrate or the lead frame by small wire bonds. The interposer substrate can also include a plurality of ball-pads coupled to the terminals by traces in the interposer substrate. An array of solder-balls is configured so that each solder ball contacts a corresponding ball-pad to define a “ball-grid” array. Packaged microelectronic devices with ball-grid arrays generally have lower profiles and higher pin counts than conventional chip packages that use a lead frame.
Packaged microelectronic devices are typically made by (a) forming a plurality of dies on a semiconductor wafer, (b) cutting the wafer to singulate the dies, (c) attaching the individual dies to an interposer substrate or lead frame, (d) wire-bonding the bond-pads on the die to the terminals of the interposer substrate or lead frame, and (e) encapsulating the die with a molding compound. It is time consuming and expensive to mount individual dies to the interposer substrates or the lead frames. Also, it is time consuming and expensive to wire-bond the bond-pads to the terminals and then encapsulate the individual dies. Moreover, the handling processes for attaching individual dies to interposer substrates or lead frames may damage the bare dies. Packaging processes, therefore, have become a significant factor in producing semiconductor devices and other types of microelectronic devices.
Another process for packaging devices is wafer-level packaging. In wafer-level packaging, a plurality of dies are formed on a wafer, and then a redistribution layer is formed over the dies. The redistribution layer has a dielectric layer, a plurality of ball-pad arrays on the dielectric layer, and conductive traces in the dielectric layer that are coupled to individual ball-pads of the ball-pad arrays. Each ball-pad array is arranged over a corresponding die, and the ball-pads in an array are coupled to corresponding bond-pads of a die by the conductive traces. After forming the redistribution layer on the wafer, a highly accurate stenciling machine deposits discrete masses of solder paste onto the individual ball-pads. The solder paste is then reflowed to form small solder balls or “solder bumps” on the ball-pads. After forming the solder balls, the wafer is singulated to separate the individual microelectronic devices from one another.
Wafer level packaging is a promising development for reducing the cost of manufacturing microelectronic devices. By “pre-packaging” the individual dies with the redistribution layer before cutting the wafers to singulate the dies, sophisticated semiconductor processing techniques can be used to form smaller arrays of solder balls. Additionally, wafer-level packaging is an efficient process that simultaneously packages a plurality of dies to reduce costs and increase throughput.
Although wafer-level packaging offers many advantages, the process typically produces devices with exposed or “bare” dies. One concern of wafer-level packaged microelectronic devices is that the bare dies may be chipped or damaged in post-packaging processes. For example, the devices are often handled individually in JEDEC trays, test sockets and tape dispensers for post-packaging, testing and shipping. Another concern is that particles can become embedded in the redistribution layer during post-packaging processes. As a result, wafer-level packaged microelectronic devices may be damaged or rendered inoperative because of post-packaging handling.